Planescape: Torment
Aethelgrin and Tegar'in the Thrice Damned are abishai on leave from current assignments in Baator. They knew the Nameless One hundreds of years ago and are disappointed he doesn't remember.
As fiends, they are on the side of the baatezu, not the tanar'ri.
(Aethelgrin and Tegar'in the Thrice Damned)